When Michael Keenan ran into a burning building to save a dog — a dog that didn’t even belong to him in a home where he didn’t live — his friends and family weren’t surprised.

The San Francisco man had already proven himself a hero six years earlier when he jumped into San Francisco Bay to save two strangers who had driven their car into the water. He knocked out the back window of their fully submerged car and pulled a woman to safety, but her husband died before he could be rescued.

Now Keenan, 43, is fighting for his life at San Francisco General Hospital. He’d safely escaped the fire at a friend’s house Tuesday morning but then did something firefighters warn people never to do: He ran back inside the house. He managed to rescue his friend’s dog but suffered burns to 80 percent of his body.

On Thursday, Keenan went in for the first of what are likely to be many surgeries, and doctors have told his friends and family that his chances of survival are 50-50.

“I know a lot of people may judge him for going back in for a dog, but it’s just part of his nature. It’s just courage,” said Frank Hsieh, a San Francisco firefighter who has known Keenan for about 20 years. “He just acts. He’s selfless. The guy’s a total hero.”

The dog, a 10-year-old Jack Russell terrier named Bobby, also is fighting to live. After Keenan pulled him from the fire, Bobby — with serious burns to his legs and the bottom of his feet — ran away, making it several blocks to the home of his owner’s friend, from where he was taken to a local pet hospital.

Keenan, an artist, had been house-sitting for Janet Gerl, a close friend, at her home on Bonita Street in Russian Hill when the fire started. He recently had returned to San Francisco after living in Los Angeles for a couple of years and was waiting for renovations to be complete at an apartment he would be renting.

The fire started at about 2:30 a.m. near the front of the house. Keenan ran safely out of the house, then ducked back into the flames when he realized the dog was still inside, according to the Fire Department. When Keenan came back out of the house, he was engulfed in flames. He was still conscious when he arrived at the hospital and talked to medical personnel about the dog, authorities said. The cause of the fire is still under investigation.

Since the fire, Keenan has been in an induced coma, friends and family said. The worst burns are to his hands and feet, and his body has been swollen by intravenous fluids. He underwent surgery Thursday to remove dead skin from his hands and arms and to replace it with cadaver skin.

The dog isn’t in much better shape. Bobby is in intensive care at Pets Unlimited, and swelling has left him blind, although it’s too soon to say whether the eye damage is permanent, said veterinarian Dr. Megan Moses.

Friends said a lot of people might question whether the risk Keenan took to save a dog was worth it. But Keenan, they said, probably never gave it a second thought.

“I’m sure he was concerned for the safety of the animal and what the owner might go through if the dog wasn’t saved,” said friend David Stefani of San Francisco. “He was not thinking about himself first, that’s for sure.”

And he wasn’t thinking of himself in December 2001 when he dove into San Francisco Bay to save a stranger, friends said.

It was Christmas Day, and Keenan and a friend had gone to the St. Francis Yacht Club for drinks to celebrate the holiday. Outside the club, Heather Laarif was teaching her husband, Abdel, how to drive, San Francisco police said at the time. The couple had pulled their Jeep Cherokee into a parking spot at the edge of the bay, and suddenly the car jerked forward — investigators assume Abdel Laarif accidentally hit the accelerator. The car tumbled into the water.

Keenan saw the accident as it happened, said Hsieh, who was part of the firefighter battalion that responded to the incident. Keenan grabbed a wrench from someone at the club and jumped into the 50-degree water, Hsieh said. He used the wrench to smash the back windshield of the Jeep and pulled out Heather Laarif. The water was so treacherous that it was hours before authorities were able to retrieve the Jeep.

“He must think he’s Spider-Man or something. He’s clearly got the hero instinct in him,” said Keenan’s brother, Jim Keenan. “He picked the wrong job. He’s an artist, and he should be a cop or a fireman or something.”

Longtime friend Stacie Krajchir of Los Angeles agreed: “I want to tell him when he wakes up, ‘If you wanted to be search-and-rescue, why didn’t you just join the damned fire department?’ ”

If you’ve read this and want to know more, here’s what we heard from a friend of Michael’s who barked in to let us all know more about his condition and what we can do to help:

His condition is very serious. We are all hopeful but even in the best scenario his recovery and rehabilitation will be a long struggle. We are so grateful that the Chronicle ran his story today and directed readers interested in making contributions to the blog we have set up for him:

So far today we have received nearly two dozen emails from dog lovers interested in making contributions.

I am so moved by the offers of support from people who do not even know Michael. I wanted to find out from you if there is an appropriate place on Dogster for this story. Michael does not have financial resources and I think tapping the support of the community is a time sensitive matter.

If you want to send a donation directly to the fund to help Michael, here is the information:

An account has been established at Wells Fargo Bank to support Michael through his rehabilitation. If you are interested in making a contribution to support Michael, you may:

Visit any Wells Fargo Bank and ask to contribute to the Michael J. Keenan Support Fund and they will be able to assist you.